This invention relates generally to recording methods and apparatus; and, more specifically, to such methods and apparatus which utilize liquid ink as a marking medium.
The advantages of liquid ink recorders are well known. One advantage is the ability to mark on low cost recording media, such as paper. Another advantage is the instantaneous display of the ink markings on the recording media.
However, most such recorders are basically mechanical in nature and have certain limitations on speed of recording.
One type of prior art ink recorder employs a retractable ink-fed stylus which deposits ink only during contact with the recording medium. This contact is controlled by electrical information signals and is not generally useful in high speed operation.
Continuous contact recorders wherein ink is deposited only during relative movement between the stylus and the recording medium are generally limited in applicability to situations where oscillographic traces are to be recorded.
Other recorders utilize a plurality of capillary-fed styli to supply ink near to the recording surface. Flow to the recording surface is effected by electrical signals applied between a particular stylus or group of styli and a conductive backing electrode which lies in the entire recording zone behind the recording medium. This usually results in a capacitive buildup of charges on the styli which causes erratic operation.
Ink recorders as described in U.S. Pat. Nos. 3,289,211 and 3,375,528, assigned to the same assignee as the instant application, eliminate many of the speed limiting factors. However, these recorders employ a recording head which deposits ink in unitary, integral amounts and must be moved physically across the recording medium in order to record an entire line of information. This operation makes high speed recording difficult.
Still other liquid ink recorders operate on the ink jet principle wherein droplets of liquid ink are jetted from an orifice and directed to a recording surface, such as paper. In some ink jet systems, the droplets are ejected from the orifice by pressure. In other ink jet systems, the ink is drawn from the orifice by an electric field. In still other ink jet systems, a stream of liquid ink flowing from a nozzle is broken into droplets by vibrations.
The droplets can be charged and then selectively electrically deflected away from the recording surface or to a different point on the recording surface. Controlling the droplets so as to make the desired markings on the recording surface requires sophisticated and oftentimes complicated controls. A more direct method of marking on a recording surface is desirable.
U.S. Pat. No. 3,738,266, Maeda et al, describes an electronic printing device wherein ink is held in a finely-meshed screen of dielectric material. The ink is charged to one polarity, and the screen is brought adjacent a recording surface, such as paper. The other side of the recording surface is then imagewise charged with an electrostatic printing tube.
The field between the charges is sufficiently strong to cause the ink within the field to move from the screen and to cling to the recording surface. The method of Maeda et al requires placing charges of one polarity on one side of the recording material while the charged ink is on the other side of the material. Because of the thickness of the recording material, a relatively high potential is needed, requiring biased rollers and the like. A method for direct marking while avoiding the use of high potentials is desirable. A still greater disadvantage of Maeda is the loss of image sharpness inherent in applying the signal information charges to the side of the recording material opposite the ink applicator.